1. Field of the Invention
The present invention relates to an optical semiconductor apparatus or optical receiver which can eliminate polarization dependency and an optical communication system utilizing the same in the field of optical frequency division multiplexing communication, for example. The problem of polarization dependency arises because a receiving sensitivity fluctuates due to changes in the polarization state of signal light on the receiver side.
2. Related Background Art
In recent years, increased transmission capacity in the field of optical communications has become desirable, and the development of optical FDM communication, in which signals at a plurality of optical frequencies are multiplexed in a single optical fiber, has been advanced.
There are two kinds of optical FDM communication methods, which are classified by the type of light signal used in the receiving technique. One method is a coherent optical communication in which a beat signal is produced between signal light and light from a local oscillator to obtain an intermediate-frequency output, and that output is detected. The other method is one in which only light at a desired wavelength or optical frequency is selected by a tunable filter, and the thus-selected light is detected. The latter method, known as an optical-frequency changeable filtering method, will be described.
The tunable filter may comprise one of a Max-Zehnder type, a fiber Fabry-Perot type and an acousto-optic (AO) type, which have been respectively developed, but each has drawbacks.
In the Max-Zehnder filter type and the fiber Fabry-Perot filter type, the transmission bandwidth can be relatively freely designed and a width of several A can be obtained, so that the frequency multiplicity of optical FDM communication can be increased. Further, there is a great advantage in that the polarization state of signal light does not adversely affect the quality of signal receiving. An example of a Max-Zehnder type filter is disclosed in K. Oda et al. "Channel Selection Characteristics of Optical FDM Filter", OCS 89-65, 1989. An example of a fiber Fabry-Perot type filter is disclosed in I. P. Kaminow et al. "FDMA-FSK Star Network with a Tunable Optical Filter Demultiplexer", IEEE J. Lightwave Technol., vol. 6, No. 9, p. 1406, September, 1988. Those filter types, however, have the disadvantages that considerable light loss exists and that downsizing of a receiver device is difficult because the integration of a semiconductor photodetector and the filter is impossible.
In the AO modulator filter type, the receiving control is easy since the transmission bandwidth is large, e.g., several tens of .ANG., but the multiplicity of transmitted wavelengths cannot be increased. An example of an AO modulator type filter is disclosed in N. Shimosaka et al. "A photonic wavelength division/time division hybrid multiplexed network using accoustic tunable wavelength filters for a broadcasting studio application", OCS 91-83, 1991. This filter type, however, has the drawbacks that light loss exists, that the integration with a semiconductor photodetector is impossible and that polarization control of signal light is necessary because the polarization state of signal light adversely affects the quality of signal receiving.
On the other hand, in a semiconductor filter type, e.g., a distributed feedback (DFB) filter provided with a diffraction grating formed in a light guide layer for single longitudinal mode operation, the transmission bandwidth can be narrowed (e.g., by several .ANG.), the optical amplification function (approx. 20 dB) exists, the multiplicity of transmitted wavelengths can be increased and the minimum receiving sensitivity can be improved (i.e., the minimum receiving intensity can be reduced). An example of a semiconductor type filter is disclosed in T. Numai et al. "Semiconductor Tunable Wavelength Filter", OQE 88-65, 1988. Further, this type of filter can be formed with the same material as a semiconductor photodetector, so that integration and downsizing are feasible.
From the foregoing, the suitability of a semiconductor DFB type optical filter for optical FDM communications is clear.
The DFB filter as shown in FIG. 1, however, has polarization dependency, which results from the fact that tuned wavelength (selected wavelength of light that can be transmitted through DFB filter) for light (TE mode) having an electric field component parallel to the layer surface of the device is different from tuned wavelength for light (TM mode) having an electric field component perpendicular to the layer surface of the device. The fact is caused by the following phenomenon. Since effective indices of the waveguide for TE and TM modes are different, the Bragg conditions of the diffraction grating: EQU .lambda.=2n .LAMBDA./m
(.lambda.: wavelength of light, n: effective index, .LAMBDA.: pitch of diffraction grating, m: integer or order of diffraction grating) deviate from each other between those TE and TM modes. In the filter shown in FIG. 1, there are arranged a waveguide 41, a grating 42 formed in the waveguide 41, three electrodes 43, 44 and 45 separated from each other along the light propagation direction, an active layer 46 and anti-reflection films 47 and 48 deposited on both opposite end surfaces. The electrodes 43 and 45 at opposite end portions (active regions) serve to make a gain, change the refractive index of the waveguide 41 by changing carrier densities therein and change the wavelength reflected by the grating 42 in a distributed manner. The electrode 44 at a central portion (a phase adjusting region) changes the carrier density distribution therein to control the refractive index, and thus changes the phase of light propagated through the waveguide 41 to achieve the wavelength tuning in a wider range.
In general, the index n for TM mode is smaller than the index for TE mode, and hence the tuned wavelength for TM mode is shifted toward a shorter wavelength side, compared to the tuned wavelength for TE mode. Therefore, if the tuned wavelength of DFB filter is adjusted so that its gain is maximum, for example, for TE mode, the transmission intensity of filter changes with time since the TE mode component varies when the polarization plane of signal light is rotated during transmission in the optical fiber. As a result, the received intensity varies with time, and a reduction of receiving sensitivity and an increase in error rate are caused. In the worst case, signal receiving is hardly achieved if all the signal is coupled to the DFB filter in TM mode.